How to Get Started with Reactive Transport Modeling in OGS6

1

Hello everyone,

Recently, I have been learning the Reactive Transport in OGS6 and have encountered several difficulties in understanding and applying it. I am already familiar with the numerical framework and general workflow of OGS, but I am still a beginner in geochemical reaction modeling. I would like to seek advice from the community to better understand the related concepts and modeling practices.

While reviewing the official benchmark cases, I noticed the following:

  • In the “Sorption of U(VI) in porous media”, the sorption process is modeled using the Surface Complexation approach. In addition, the .prj file includes a parameter “g_Tot = 10388”, but it is unclear to me what this value specifically represents and how it is determined.

  • In the “Radionuclides migration in Opalinus clay” , the sorption process is described using the mechanistic approach.

Although both cases aim to model sorption processes, these two approaches differ significantly in their theoretical background and the way they are defined in the .prj file, especially in the chemical reaction and sorption settings. As a beginner, I find this quite confusing:

  • What are the fundamental differences between these two approaches in terms of theory and application? Under what circumstances should one use the Surface Complexation approach versus the mechanistic approach?

  • Are there any recommended learning materials or tutorials that could help me systematically understand these two modeling strategies and their implementation in OGS?

I have studied all the reactive transport-related benchmarks and also read the book “Computational Hydrology III: OGS5#IPhreeqc Coupled Reactive Transport Modeling”. However, I am still confused about these issues and have not fully understood the logic behind the configuration of reactive models in OGS. As a result, I find it difficult to flexibly apply these techniques to other similar research problems. These questions may seem basic to experienced users, but they are still challenging for me. I would sincerely appreciate any guidance, explanations, or recommendations from the community.

Thank you very much for your time and help! I hope to gain a clearer understanding of the modeling concepts and methodological choices behind reactive transport simulations in OGS6.

2

Quick answer: the values are for PHREEQC, not for OGS (they are hosted in the command grouped in <user_punch> which is typical of phreeqc Basic language.

The 10388 parameter is here specifically the Kd (according to the comment of the same line)
<statement>g_Tot=10388 # Kd = (mol_sorb/g_tot)/(mol_aq) --> Kd in l/g = m³/kg</statement>

1 Like
3

Hi lurpi,

Thank you very much for your detailed explanation!

I just had a preliminary look at the official PHREEQC manual, and indeed, it provides explanations regarding both the Surface Complexation approach and the mechanistic approach. If you could suggest any additional references or learning materials—particularly those related to sorption problem—I would greatly appreciate it. As someone new to this area, I find it difficult to know where to start and how to build a coherent understanding of the key concepts.

Thanks again for your time and kind help!

4

Hello,

I went through this book

“Geochemistry, groundwater and pollution, second edition”
Author: C. appelo, D. Postma
Chapter 6 and 7. Althogh sorption is in chapter 7, chapter 6 helps to understand detail of chapter 7.

I hope it will help.

1 Like
5

Hi mmhassan,

Thank you very much for the recommendation! I really appreciate your suggestion — I’ll definitely go through this book carefully. It looks like a great resource to deepen my understanding of sorption modeling.